1. Field of the Invention
The present invention relates to a device which performs a bending process on a plate workpiece, and, specifically, to a plate bending machine provided with a control mechanism to precisely position upper and lower bending tools by eliminating play or backlash between them before abutment.
2. Description of the Prior Art
A press brake is commonly known as a device which performs a bending process on a plate workpiece. In a conventional press brake, an upper table (ram) with a vertically operating format which supports an upper bending tool in a press brake, the upper table descends, and when it performs the bending operation on the workpiece the play in each of the connected parts, for example, the play between the connecting rod and the crankshaft, or the play between the screw and the ram must be taken up, so a cylinder or spring is used as a counterbalance.
However, when a cylinder is used, a motive source is required, and when a spring is used, a long spring is necessary in relation to the spring constant to cover the entire stroke, and in the case where the upper frame positioning is carried out in the vicinity of the bottom dead point there occurs the problem that positioning with good precision is not possible.
Further, in order to adjust the height of the upper table to accommodate the change in height of the top and bottom bending tools and to accommodate the change in thickness of the workpiece, the left and right side sections of the upper table are mounted in a manner to freely adjust the vertical position through the medium of a bushing or the like in the guide posts.
Accordingly, the problem arises that the positional control of the upper bending tool relative to the lower bending tool cannot be performed with high precision because of the play.